1. Field of the Invention
This invention relates to a multi-stage amplifier device and a method for producing the same, which is used as a microwave low-noise amplifier for a satellite-broadcast receiving converter, a wideband amplifier for a measuring equipment or the like.
2. Related Background Art
In association with a recent rapid development of an information net work system, a satellite communication and broadcasting system have been intensively required, and a frequency band of the system has been also required to be further shifted to a higher one. In order to satisfy these requirements, various circuit elements such as a field effect transistor, etc. which can be operated in a high frequency band have been studied and developed. A Schottky-barrier type electric field effect transistor (MESFET) formed of compound semiconductor has been used such as a field effect transistor for high frequency band. Especially GaAs compound semiconductor transistor has been employed because it can break-through a limitation in characteristics of an Si bipolar transistor. In addition, in order to satisfy various requirements such as miniaturization, low-cost and high performance for a system, integration of a first-stage amplifying portion of a down-converter has been recently constructed as MMIC (Microwave Monolithic Integrated Circuit).
In an MMIC low-noise amplifier used in a microwave band, amplifiers at all stages thereof have been conventionally formed of the same FET (Field Effect Transistor), that is, the front and rear stages of the MMIC amplifier are not designed so as to have different FET constructions. In order to attain a low noise amplifier, an FET having a gate structure which is designed in self-alignment with a source and a drain has been practically used.
In order to reduce a noise figure of a FET, it is important to increase transfer conductance gm of the FET and lower gate-source capacitance Cgs of the FET, or to lower source resistance Rs. In order to increase the transfer conductance gm, and to lower the gate-source capacitance Cgs, the gate length of the FET is mainly required to be set to 0.25 .mu.m or less. On the other hand, in order to lower the source resistance Rs, it is mainly required to approach an n.sup.+ contact region to the gate as near as possible. If the gate is formed in a self-alignment structure, the gate length could be shortened, and the n.sup.+ contact region and the gate could be formed in such a manner that an interval therebetween is remarkably shortened with high reproducibility irrespective of alignment accuracy of an exposing device. The lowering of the source resistance Rs is also effective for the increase of gm and the lowering of an equivalent noise resistance Rn.
As described above, in the conventional MMIC low-noise amplifier used in the microwave band, the amplifiers at all stages are designed using FETs having the self-alignment structure.
However, when the gate is formed in the self-alignment structure as described above, an n.sup.+ contact region at the drain side is also nearly approached to the gate, like the n.sup.+ contact region at the source side, so that a drain-gate breakdown voltage (a durable maximum voltage to be applied between the drain and the gate) will be lowered.
The MMIC amplifier has a large gain with one chip having a multistage connection. Therefore, even if an input signal power is weak at the first stage, the amplitude of the signal is more intensified as it proceeds to the subsequent stages. Therefore, there frequently occurs a case where a voltage exceeding the drain-gate breakdown voltage of the FET is unintentionally applied between the drain and the gate of the FET, and the FET is damaged. Moreover, in such a type of amplifier, a parameter representing linearity of an amplifier, such as 1 dB compression output power or IM.sub.3 intercept point, is regarded as an important index value, and a higher index value represents higher performance of the amplifier. If the voltage exceeding the drain-gate breakdown voltage of the FET is applied between the drain and the gate of the FET as described above, the linearity of the amplifier would be deteriorated, and values of the 1 dB compression output power and the IM.sub.3 intercept point would be reduced.